QRPKits BITX20A User manual

Hendricks QRP Kits
BITX20A to BITX17A
Conversion Instructions
30 November 2008
Converting your BITX20A Kit to a BITX17A Kit is not all that complex. It only requires that you
change crystals and some resonance components to values used in the BITX17A design. The basic
design for both 20 meters and 17 meters is the same, thus DC voltages and troubleshooting methods
remain the same except for obvious frequency differences.
There is a BITX discussion group available at: http://groups.yahoo.com/group/BITX20/
Doug Hendricks KI6DS web site for kit sales is at: http://www.qrpkits.com
Farhan's original BITX20 design web site is at: http://www.phonestack.com/farhan/bitx.html
Page 1 of 34

Your BITX20A to BITX17A Conversion Kit will include a decal sheet, crystals, inductors, and various
capacitors needed for the conversion.
You will be removing installed components (unless your are converting an as-yet
unfinished BITX20 kit) and installing replacement components in their place. Some
care is required to avoid damaging the PCB in the un-soldering and re-soldering
process. Solder wick (or a section of braid salvaged from a length of coaxial cable
shield) helps to remove excess solder, as does a spring-driven “solder sucker” tool.
The round type of wooden toothpicks can also be used to push through heated holes
where components have been removed. This opens the holes for installation of new
components.
Page 2 of 34

Conversion Parts List:
Part Number 17 Meter Parts 20 Meter Parts Part Description
X-1 12.96 MHz 11.0 MHz Crysta
X-2 12.96 MHz 11.0 MHz Crysta
X-3 12.96 MHz 11.0 MHz Crysta
X-4 12.96 MHz 11.0 MHz Crysta
X-5 12.96 MHz 11.0 MHz Crysta
L-10 1.0 uH 1.2 uH Mo ded Choke
L-5 6.8 uH 8.2 uH Mo ded Choke
C-92 51 pf 68 pf Ceramic Capacitor
C-54 68 pf 82 pf Ceramic Capacitor
C-55 82 pf 100 pf Ceramic Capacitor
C-56 82 pf 100 pf Ceramic Capacitor
C-47A 82 pf 100 pf Ceramic Capacitor
C-47B 82 pf 100 pf Ceramic Capacitor
C-48A 150 pf 180 pf Ceramic Capacitor
C-48B 150 pf 180 pf Ceramic Capacitor
C-49A 82 pf 100 pf Ceramic Capacitor
C-49B 82 pf 100 pf Ceramic Capacitor
C-34 390 pf 470 pf Po ystyrene Cap
C-35 390 pf 470 pf Po ystyrene Cap
C-37 130 pf 220 pf Po ystyrene Cap
C-11 8.2 pf 10 pf Ceramic Capacitor
C-13 68 pf 82 pf Ceramic Capacitor
C-15 120 pf 180 pf Ceramic Capacitor
C-16 10 pf 8.2 pf Ceramic Capacitor
C-17 47 pf 56 pf Ceramic Capacitor
C-19 8.2 pf 10 pf Ceramic Capacitor
C-20 68 pf 82 pf Ceramic Capacitor
C-24 6.8 pf 8.2 pf Ceramic Capacitor
C-25 68 pf 100 pf Ceramic Capacitor
Page 3 of 34

Parts Placement Diagram:
( Drawing by KD1JV )
Page 4 of 34

BITX17A Schematic Diagram:
( Drawing by N7VE )
Page 5 of 34

Conversion Steps
Now might be a good time to check R-2. R-8, R-83, and R-91 to insure that they
are turned totally counter-clockwise. This is to make sure that the transmitter
RF drive level is at minimum, and that the MOSFET bias levels are also at
minimum. During the alignment procedure you will readjust these
potentiometers to their proper levels.
BFO Components:
[ ] Remove L-5 (8.2 uh molded inductor) marked Gray-Red-Gold-Gold
[ ] Install new L-5 (6.8 uh molded inductor) marked Violet-Gray-Gold-Gold
[ ] Clip excess lead length.
[ ] Inspect solder connections.
[ ] Remove X-5 (11 MHz Crystal)
[ ] Install new X-5 (12.96 MHz Crystal)
NOTE: Before you install the new crystal for X-5, you may want to temporarily
insert each new 12.96 MHz crystal into the BFO oscillator position (X-5) and
measure the default frequency for each crystal (measure BFO frequency at the top
of R-67). Mark each crystal with it's frequency. Now select the 4 crystals which
are closest in frequency for use in X-1, X-2, X-3, and X-4. The remaining 12.96
MHz crystal will be your X-5 crystal for the BFO oscillator. While not absolutely
necessary, this procedure may provide steeper sidebands on your crystal filter than
would a random selection of crystals.
[ ] Remove X-1 (11 MHz Crystal)
[ ] Install new X-1 (12.96 MHz Crystal)
[ ] Remove X-2 (11 MHz Crystal)
[ ] Install new X-2 (12.96 MHz Crystal)
[ ] Remove X-3 (11 MHz Crystal)
[ ] Install new X-3 (12.96 MHz Crystal)
[ ] Remove X-4 (11 MHz Crystal)
Page 6 of 34

[ ] Install new X-4 (12.96 MHz Crystal)
[ ] Clip excess lead length.
[ ] Inspect solder connections.
VFO Blocking Filter (L-10 & C-25):
[ ] Remove L-10 (1.2 mh molded choke)
[ ] Install new L-10 (1.0 mh molded choke)
[ ] Remove C-25 (100 pf Ceramic Capacitor)
[ ] Install new (C-25 (68 pf Ceramic Capacitor)
[ ] Clip excess lead length.
[ ] Inspect solder connections.
[ ] Remove C-92 (68 pf Ceramic Capacitor)
[ ] Install new C-92 (51 pf Ceramic Capacitor)
[ ] Remove C-54 (82 pf Ceramic Capacitor)
[ ] Install new C-54 (68 pf Ceramic Capacitor)
[ ] Remove C-55 (100 pf Ceramic Capacitor)
[ ] Install new C-55 (82 pf Ceramic Capacitor)
[ ] Remove C-56 (100 pf Ceramic Capacitor)
[ ] Install new C-56 (82 pf Ceramic Capacitor)
[ ] Clip excess lead length.
[ ] Inspect solder connections.
[ ] Remove C-47A (100 pf Ceramic Capacitor)
[ ] Install new C-47A (82 pf Ceramic Capacitor)
[ ] Remove C-47B (100 pf Ceramic Capacitor)
[ ] Install new C-47B (82 pf Ceramic Capacitor)
[ ] Remove C-48A (180 pf Ceramic Capacitor)
[ ] Install new C-48A (150 pf Ceramic Capacitor)
Page 7 of 34

[ ] Remove C-48B (180 pf Ceramic Capacitor)
[ ] Install new C-48B (150 pf Ceramic Capacitor)
[ ] Remove C-49A (100 pf Ceramic Capacitor)
[ ] Install new C-49A (82 pf Ceramic Capacitor)
[ ] Remove C-49B (100 pf Ceramic Capacitor)
[ ] Install new C-49B (82 pf Ceramic Capacitor)
[ ] Clip excess lead length.
[ ] Inspect solder connections.
[ ] Remove C-34 (470 pf Polystyrene Capacitor)
[ ] Install new C-34 (390 pf Polystyrene Capacitor)
[ ] Remove C-35 (470 pf Polystyrene Capacitor)
[ ] Install new C-35 (390 pf Polystyrene Capacitor)
[ ] Remove C-37 (220 pf Polystyrene Capacitor)
[ ] Install new C-37 (130 Polystyrene Capacitor)
[ ] Clip excess lead length.
[ ] Inspect solder connections.
[ ] Remove C-11 (10 pf Ceramic Capacitor)
[ ] Install new C-11 (8.2 pf Ceramic Capacitor)
[ ] Remove C-13 (10 pf Ceramic Capacitor)
[ ] Install new C-13 (8.2 pf Ceramic Capacitor)
[ ] Remove C-15 (180 pf Ceramic Capacitor)
[ ] Install new C-15 (120 pf Ceramic Capacitor)
[ ] Remove C-16 (10 pf Ceramic Capacitor)
[ ] Install new C-16 (8.2 pf Ceramic Capacitor)
[ ] Remove C-17 (56 pf Ceramic Capacitor)
Page 8 of 34

[ ] Install new C-17 (47 pf Ceramic Capacitor)
[ ] Clip excess lead length.
[ ] Inspect solder connections.
[ ] Remove C-19 (10 pf Ceramic Capacitor)
[ ] Install new C-19 (8.2 pf Ceramic Capacitor)
[ ] Remove C-20 (82 pf Ceramic Capacitor)
[ ] Install new C-20 (68 pf Ceramic Capacitor)
[ ] Remove C-28 (8.2 pf Ceramic Capacitor)
[ ] Install new C-28 (6.8 pf Ceramic Capacitor)
[ ] Clip excess lead length.
[ ] Inspect solder connections.
[ ] Remove L-7 (VFO Oscillator Coil) Remove 14 turns. This should leave 36 turns on
the core.
[ ] Re-install L-7
[ ] Remove L-6 (RF Filter Coil) Remove 4 turns. This should leave 14 turns on the core.
[ ] Re-install L-6
[ ] Remove L-4 (RF Filter Coil) Remove 4 turns. This should leave 14 turns on the core.
[ ] Re-install L-4
[ ] Clip excess lead length.
[ ] Inspect solder connections.
[ ] Remove L-1 (Antenna Filter) Remove 3 turns. This should leave 9 turns on the core.
[ ] Re-install L-1
[ ] Remove L-2 (Antenna Filter) Remove 3 turns. This should leave 9 turns on the core.
[ ] Re-install L-2
[ ] Clip excess lead length.
[ ] Inspect solder connections.
Page 9 of 34

Congratulations. You have completed converting your BITX20A to a BITX17A, and are ready to
proceed with alignment of the transceiver, including transmitter driver stages and the MOSFET power
amplifiers.
Now might be a good time to check R-2. R-8, R-83, and R-91 to insure that they are turned
totally counter-clockwise. This is to make sure that the transmitter RF drive level is at
minimum, and that the MOSFET bias levels are also at minimum. During the alignment
procedure you will set these potentiometers at their proper levels.
To proceed with transceiver alignment you will need the following tools:
●50 ohm Dummy load. This can be an actual dummy load, or a bank of non-inductive resistors
which will withstand 12 watts or more for up to one minute.
●An Oscilloscope, or Diode Voltage Probe and Voltmeter for measuring RF voltage levels at 18
MHz.
●A DC Ampere Meter capable of measuring 0 to 300 ma, and 0 to 2 amperes. There is an
alternative way to measure the 0 to 300 ma levels using voltage across a resistor. This will be
discussed in the alignment procedures.
●The Microphone that you are going to use with this transceiver, or an audio signal generator and
suitable attenuators to adjust it's output to normal microphone signal levels (nominally 40 to 50
millivolts).
●A Frequency Counter, or other means to determine VFO frequency and tuning range. A
receiver with accurate frequency readout may be suitable.
Initial Alignment Procedure:
These steps will get you on the air. A more detailed alignment method is documented in the
troubleshooting section.
1. Set the BFO and carrier oscillator frequency:
Adjust the BFO (C-72) so that when you tune across a steady carrier you can hear the zero-beat
on each side of that carrier frequency. Then adjust C-72 to a slightly lower frequency where you
can hear the upper-sideband zero-beat tone but the lower-sideband one is either gone or very
much attenuated. Now tune in an on-air SSB signal and adjust C-72 very slightly for best audio
quality.
2. Set the carrier balance:
STEP-1: With an detector probe or oscilloscope monitorin the 12.96 MHz si nal at
Page 10 of 34

R-64, carefully adjust R-66 and C-65 for minimum si nal level. There is some
interaction so you will need to re-adjust these several times in order to et the best
si nal null.
STEP-2: Move your oscilloscope probe or diode detector to pin-2 of X-4 (the junction
of X-4, C-62, and C-89), and a ain adjust R-66 and C-65 for the best carrier null
while in transmit mode (PTT operated).
3. Determine and adjust the VFO frequency and tunin ran e:
Set your Fine Tuning potentiometer to mid-range. Leave it there for the following procedure.
Since the BFO and IF sections operate on 12.96 MHz, your BITX17A VFO must tune between
18.068 – 12.960 = 5.108 MHz and 18.168 – 12.960 = 5.208 MHz. Set your Course Tuning
adjustment (the polyvaricon capacitor) fully to the left. Adjust the trimmer on the back of your
polyvaricon variable capacitor so that with the knob turned fully left (minimum capacitance)
your VFO is at or slightly above 5.208 MHz. Then rotate the polyvaricon fully to the right
(maximum capacitance) and verify that the VFO is now below 5.108 MHz. This insures full 17
meter band coverage.
4. Ali n the front-end bandpass filter:
While listening to a signal generator or received carrier beat note, carefully adjust C-14 and
C-18 for maximum received signal level. There is some interaction between these 2 capacitors,
so you may need to re-peak each capacitor adjustment several times to reach maximum
sensitivity. You should see two positions on these capacitors where the signal peaks. This
indicates that you are actually tuning to the maximum sensitivity point, instead of just
approaching resonance at the end of tuning range.
5. Adjust the transmit driver bias:
Temporarily short the microphone audio input connection. Connect an ammeter capable of
measuring approximately 200 ma in series with power to the transceiver. Key the microphone
PTT and note the idling current of your transmitter. Now slowly rotate R-8 clockwise until the
idling current has increased by 20 ma. Un-key the transmitter and remove the temporary short
across the microphone audio input.
NOTE: It is possible to measure the combined current flowing in Q-18 and Q-19 by measuring
the DC voltage across R-7 with a sensitive digital voltmeter. This is a 2.2 ohm resistor, so 20
ma through this 2.2 ohm common source resistor would occur at a voltmeter reading of 0.20 X
2.2 = 0.044 volts.
6. Adjust the transmit power amplifier bias:
Temporarily short the microphone audio input connection. Connect an ammeter capable of
measuring approximately 300 ma in series with power to the transceiver. Key the microphone
Page 11 of 34

PTT and note the idling current of your transmitter. Now slowly rotate R-91 clockwise until the
idling current has increased by 50 ma. Note the new idling current. Slowly rotate R-2
clockwise until the idling current has increased by another 50 ma. Un-key the transmitter and
remove the temporary short across the microphone audio input.
7. Drive level adjustment:
Monitor transmitter output with a 50 ohm termination and watt-meter, or dummy load and scope
or diode detector probe and DC voltmeter.
Temporarily connect an audio signal generator to the microphone audio input connection (if
R-92 is installed you will need to use capacitive coupling to keep DC out of the signal
generator). Adjust the signal generator for 1000 Hz and 40 to 50 mv output level.
NOTE: The time honored method of speaking a prolonged “Aaaaaaaaaaah” into the
microphone will also work if you do not have an audio signal generator.
Key the transmitter and slowly rotate R-83 to the right while watching the transmitter RF output.
An RMS voltage reading of 22.4 volts indicates power output of 10 watts. If your equipment is
reading in peak volts, 10 watts will be at an indication of 1.4 * 22.4 = 31.3 volts.
This completes the initial adjustment and alignment of your BITX17A transceiver kit.
The following document sections provide additional information for education and for troubleshooting.
The parts placement drawing and schematic drawing have been duplicated in this section so you will
have a copy to write notes on as a future reference for your particular BITX17A transceiver.
Page 12 of 34
50 OHM VOLT GE CH RT
Watts Vo ts RMS Vo ts PEAK
1 7.07 9.9
2 10 14
3 12.25 17.15
4 14.14 19.8
5 15.81 22.14
6 17.32 24.25
7 18.71 26.19
8 20 28
9 21.21 29.7
10 22.36 31.3
11 23.45 32.83
12 24.49 34.29
13 25.5 35.69
14 26.46 37.04
15 27.39 38.34

Parts Placement Drawing:
Drawing by KD1JV
Page 13 of 34

Schematic Diagram:
Schematic by N7VE
Page 14 of 34

Detailed Alignment Method:
BFO/Carrier Oscillator:
There are several ways to adjust the BFO/Carrier Oscillator frequency.
1. Rough alignment may be made by adjusting C-72 while listening to a received SSB
signal on 17 meters. This is a two-handed operation as you have to use the VFO Fine
Tuning to keep the VFO frequency set for best intelligibility while searching for the best
quality audio with C-72.
2. Instrument alignment is done by monitoring carrier attenuation while adjusting the BFO/
Carrier Oscillator relative to the crystal filter sideband shape factor. The oscillator
frequency needs to be 24 to 40 db down the lower side of the filter passband.
●Disconnect the microphone or short it's audio output so that there is no
audio coming through the microphone amplifier.
●Set up an oscilloscope or RF detector and voltmeter to monitor the
transmit 2nd IF amplifier output (the exposed lead on R-45, near Q-10).
●Connect a frequency counter to the exposed lead of R-67 to monitor BFO/
Carrier Oscillator frequency.
●Operate the PTT switch and un-balance the balanced modulator by
turning R-66 until you have a measurable signal. Un-key the transmitter.
●Key the transmitter again and adjust the BFO/Carrier Oscillator frequency
(C-72) for maximum signal level, note the frequency on the counter, and
then adjust C-72 for a lower frequency and a 24 to 40 db decrease in
monitored signal level. Un-key the transmitter. The BFO/Carrier
Oscillator frequency needs to be just outside the lower frequency edge of
the crystal filter for USB operation.
●Disconnect the frequency counter, key the transmitter and re-check to see
that you are still at the same signal attenuation point on your scope or
voltmeter. Un-key the transmitter. This checks to make sure that your
counter was not pulling the BFO/Carrier Oscillator off frequency by any
significant amount.
●Key your transmitter and adjust the modulator balance (R-66 and C-65)
for minimum RF indication. Un-key your transmitter.
●Re-connect your microphone or remove the short from it's output.
There are several additional ways to adjust the BFO/Carrier Oscillator frequency. If you have
the knowledge and equipment it might be an interesting exercise to try these methods as well.
However, even the Rough Alignment method outlined above will usually provide adequate
setting for good reception and transmission.
VFO:
VFO components provided in the kit have been chosen in an attempt to insure that your tuning
Page 15 of 34

range will be within the 17 meter band. However, this is not guaranteed. You should check the
VFO frequency with a counter connected the exposed lead of R-27, or use a well calibrated
receiver. Trimmer capacitors on the back of the main tuning capacitor will provide some
adjustment but you will need to select which capacitor sections are connected to set the
minimum & maximum tuning limits and the tuning range.
Polyvaricon Capacitance Trimmer Open Trimmer Closed
Section Range Tuning Range Tuning Range
========= ========= =========== ============
Section-A 4 to 80 pf* 60 KHz 58 KHz
Section-B 5 to 160 pf* 124 KHz 120 KHz
Sections A+B 9 to 240 pf* 155 KHz 147 KHz
Page 16 of 34
Illustration 1: *VFO Tuning Capacitor Parameters(K7HKL
Photo)

Since the IF frequency is 12.96 MHz, the VFO needs to tune 12.96 MHz below your desired tuning
range on 17 meters (18.000 -12.96 = 5.04 MHz). You can use this method to calculate the frequency of
the VFO for your particular tuning selection.
Receiver & Transmitter RF Filter:
The easiest way to tune the RF
input filter is to peak C-14 and
C-18 in receive mode for loudest
signal while listening to a
relatively constant carrier from
your antenna. This provides both
receive and transmit mode
alignment of this filter.
Transmitter Driver
There is no tuning involved in adjusting the transmitter drivers section, but you do have to set
the bias adjustment. Drive level adjustment will be done in the RF PA alignment section.
1. Connect a 2 to 3 ampere DC ammeter in series with the +12 volt line to your
transceiver.
2. Disconnect the microphone or short it's output so there will be no audio entering
the microphone amplifier.
3. Key the transmitter and note the idling current. Un-key the transmitter.
4. Key the transmitter and slowly adjust R-8 for an increase of 20 ma. Un-key the
transmitter.
NOTE: You can alternatively monitor the voltage drop across R7 to
measure idling current in the PA driver amplifier. This is a 2.2
ohm resistor so 20 ma of current would equal a voltage drop
of (0.02 X 2.2 =) 0.044 volts.
Page 17 of 34

RF PA
There are no tuning adjustments in the RF PA section, but you do have to adjust the idling
current on the two IRF510 MOSFET devices and then set the drive level for best output
linearity. To do this you will need an ammeter with a capability of reading 2.5 or 3 amperes.
PA Idling Current Adjustment:
1. Connect an ammeter in series with the power lead that supplies 12 volts to your
transceiver.
2. Disconnect the microphone, or short it's audio output so that no audio is going to
the microphone amplifier.
3. Key the transmitter (use a dummy load please) and note the idling current. Unkey
the transmitter.
4. Key the transmitter and slowly adjust R-91 until this current increases by 50 ma.
Unkey your transmitter. Note the new idling current.
5. Key the transmitter and slowly adjust R-2 until the idling current increases by
another 50 ma. Unkey the transmitter.
Drive Level Adjustment:
RF output level can be adjusted for over 10 watts in some transceivers, but
compression of the RF envelope usually starts at about 10 watts. To preserve
output linearity you should set the RF output level no higher than the point where
RF compression becomes apparent. As you monitor the RF output and
simultaneously increase the drive level, you should see the output rise fairly fast,
with the rate of rise decreasing after you pass some point on the adjustment. This
is the setting where RF compression starts. For best transmitted signal linearity
you should set the drive level just below this start of compression point.
1. You can insert an audio tone (approximately 50 mv) via the microphone jack of
your transceiver or use the time-honored method of saying a long
“Aaaaaaaaaaaaaah” while monitoring the RF output with a scope, diode detector
and voltmeter, or watt meter.
2. Key your transmitter and insert the tone while monitoring the RF output and
slowly adjusting the RF drive pot (R-83) for an increase in output. Identify the
adjustment position where rate of output increase seems to slow. Back the drive
level back down by ½watt or so. This is your position of best output signal
quality. Un-key your transmitter.
NOTE: Do not keep the transmitter keyed for more than a few seconds with
a tone input. Steady tones exceed the duty cycle designed into the
RF PA amplifiers and their heat sinks, and may damage the
IRF510 MOSFETS.
Page 18 of 34

Troubleshooting Section
This section contains isolated sections of the total schematic, voltage charts, and a few words regarding
each section. Voltages shown are based on 12.0 volts supply for the transceiver.
Receive Audio Amplifier
NOTE: The following voltage measurements are made in receive mode
LM386 Voltages Q-16Voltages Q-25 Voltages
Pin-1 1.34 VDC Collector 10.97 VDC Collector 5.94 VDC
Pin-2 0.0 VDC Emitter 5.26 VDC Emitter 0.0 VDC
Pin-3 0.0 VDC Base 5.94 VDC Base 0.67 VDC
Pin-4 0.0 VDC
PIN-5 1.34 VDC
Pin-6 5.72 VDC
Pin-7 11.5 VDC
Pin-8 5.63 VDC
C-47 is an artifact of early PCB layout and testing. No capacitor should be inserted in this position.
Page 19 of 34

Microphone Amplifier
Q-15 Voltages (transmit mode)
Collector 6.29 VDC
Emitter 1.63 VDC
Base 2.34 VDC
R-92 provides DC voltage to operate an Electret type microphone, which uses a thin metalized mylar
foil capacitive transducer driving an internal FET amplifier. Audio AC voltage is amplified and
appears across R-92, where it is coupled through C-60 to the base of Q-15. If you are not using this
type of microphone you should not install (or should remove) R-92. Electret microphones are common
in inexpensive audio devices (tape recorders, etc.) or can be purchased directly from component
suppliers. Most inexpensive computer microphones are Electret types.
Audio input level for transmitter testing should be 50 mv or less. Use capacitive coupling (10 mfd) if
R-92 is in place, or temporarily lift one end of R-92 to avoid sending DC power into your test oscillator.
Microphones used with this transceiver should be capable of providing 40 to 50 mv of audio output. If
using a Electret microphone you will need R-92 in circuit to power the microphone. Overdriving the
audio input may cause harmonics of audio frequencies to be generated in the balanced modulator.
Page 20 of 34